Many people consider their pets to be part of their family. However, household and farm pets can cause mild health problems in humans, such as allergies, or even carry and transmit diseases. Some pets are more likely than others to carry diseases, including reptiles (turtles, lizards, and snakes), baby chicks, and ducklings, which can all transmit salmonella to humans. Proper hygiene and handwashing around pets can help to eliminate some of these risks.

Some people are more likely than others to get diseases from pets, including the elderly, pregnant women, infants, children under the age of 5, people with organ transplants, people undergoing cancer treatment, and people with HIV/AIDS.

Pets that have been treated with pesticides or wear flea collars can expose humans to the health risks of pesticides.

Cats may carry toxoplasmosis, a parasitic disease which can be transmitted through cat feces in litter boxes. If pregnant women are exposed to cat litter boxes, they may get toxoplasmosis, which can cause miscarriages and premature births in pregnant women, and birth defects in fetuses. If cats have fleas, they may get bacteria from the fleas that can turn into cat-scratch disease. This disease causes fever, fatigue, headache, and swollen glands in humans if they are bitten or scratched by infected cats.

Most cats and dogs are vaccinated against rabies but if they are not vaccinated, they can spread that usually deadly disease. They can also carry and transmit hookworms, ringworms, roundworms, tapeworms, and salmonella. If dogs have ticks, they can transmit Lyme disease and Rocky Mountain spotted fever.

Pet parakeets and parrots may have psittacosis, or "parrot fever," which can cause humans to have flu-like symptoms such as fever, chills, headaches, or muscle aches.

Farm animals can transmit rabies, ringworm, salmonella, West Nile virus, and Q fever, a bacterial disease associated with cattle, goats, and sheep. Pet hamsters and mice can carry rat-bite fever, a bacterial illness. Fish and their aquarium water can carry salmonella and bacteria that may cause human illness.